Some Dev Ideas
1. A PvP mode using the old Heroes' Battle maps where already dead bosses are scattered about. Every player on the map will have 8 signet of captures they can use to capture the bosses' elite skills before (or while) attacking the other players. When another player dies, signet of capture works on them too.
2. A Guild Wars construction kit, which basically would allow players to create new questlines (and possibly missions) to keep the game going. There could be an NPC that keeps track of the most popular, newest, and tagged players for you, and gives a standard reward depending on how difficult the quest was (as voted on by the players). I'm not imagining any new physical llandmarks or anything, but spawning a few monsters and npc's here and there might not be too difficult.
3. Heroes/Henchmen in AB, JQ, FA, etc... to cut down on waiting times.